Substance use disorders cause serious problems, both for the affected individuals and for society in general. Despite intensive research, however, a reliable laboratory test for diagnosing a patient as having, or for being at risk for developing, such conditions has not been developed. Such diagnoses are still generally made clinically, on the basis of observed behavior. Given the difficulties of defining normal experience and behavior and the lack of reliable objective indicators, it is not surprising that to date systems of diagnosis in psychiatry have been less than satisfactory. A reliable laboratory test would be of practical value in everyday clinical practice, for example, in assisting doctors in prescribing the appropriate treatment for their patients. Thus, methods of identifying subjects that have, or are at risk for developing, substance use disorders are needed.